Social influences, such as family environment and peer characteristics, are relatively robust, proximal predictors of adolescent substance use. Results from the original project period indicate reductions, on average, in adolescents' substance use following group-based treatment delivered in a community setting; readiness to change predicts substance use over follow-up; and reduction in substance using peers is associated with lower substance use. To follow-up on these findings, 200 adolescents (and their parent) will be recruited from community-based Intensive Outpatient Treatment (IOP) for substance use to participate in a longitudinal study examining social influences (i.e., group-based treatment, peer network, family environment) associated with treatment outcome. Adolescents complete assessments at baseline (treatment entry), 6-, and 12-months covering substance use, personal social network, and psychiatric status. Adolescents also complete monthly follow-ups to track changes in their social network, and new to the continuation, complete ratings after each IOP session to examine associations between group-based treatment (e.g., session norm regarding abstinence) and readiness to change substance use. Parents complete assessments at baseline (treatment entry), 6-, and 12-months. Study aims are to: (1) track changes in adolescent's readiness to change substance use, a sensitive and proximal predictor of substance use behavior, during and after IOP treatment to identify common group-based treatment factors associated with adolescents' readiness to change and substance use; (2) examine peer selection and peer influence as mechanisms underlying changes in adolescents' substance use; and (3) identify family environment characteristics that facilitate or impede reductions in adolescents' substance use. These study aims will provide important new longitudinal data relevant to the ecologically valid study of short-term outcomes of group-based treatment for adolescents, and social network influences on adolescents' treatment outcomes. Study findings will be used to develop socially- based interventions that promote healthy peer and family relations in treated adolescents. PUBLIC HEALTH RELEVANCE: Social influences, such as participation in group-based treatment for substance abuse, peer social network, and family environment, are relatively robust predictors of adolescent substance abuse treatment outcome. However, little is known regarding the association between common therapeutic factors in group-based treatment (e.g., group norm regarding abstinence) and treatment outcome, as well as how an adolescent's social network composition and structure are related to treatment outcome. This continuation project addresses these gaps in knowledge using intensive data collection in a naturalistic follow-up design to examine social influences on treated adolescent's substance use. Study results have potentially important implications for guiding the development of social network interventions for youth to enhance and maintain treatment gains.